1. Field of the Invention
The present invention relates to a plastic lens to be used for an optical system (laser optical system) for a laser light source, and more particularly to a plastic lens suitable for use in a lens of a laser scanning optical system of a laser beam printer or a pickup optical system for an optical disk.
2. Related Background Art
A lens made of a glass material has been used in the optical system of the optical pickup and the optical system of the laser beam printer.
When a lens to be used in the optical system for the laser light source (hereinafter referred to as a laser optical system) is made of polymethyl methacrylate resin and a temperature and a humidity which are environmental conditions of the optical system are changed (for example, 20.degree. C., 50% to 50.degree. C., 95%), an optical performance usually varies so that a focus point significantly varies.
The variation of the focus point greatly depends on a lens shape, particularly a ratio D/H of a lens thickness D and a lens outer diameter (or lens height) H.
When the lens for the laser optical system is made of polycarbonate resin or polystyrene resin, the variation of the focus point due to the change of humidity of the environmental condition is smaller.
However, when beam shapes before and after the laser beam passing through the lens are compared, the beam shape of the laser beam before it passes through the lens is generally circular while the beam shape of the laser beam after it passes through the lens is elliptic or divided into two parts like a figure "8".
On the other hand, when the laser beam passes through a lens made of polymethyl methacrylate resin, the variation of the focus point due to the change of humidity is large but the change of the beam shape before and after the light beam passes through the lens is not significantly changed.